It not even necessary to mention that Allah plays a central point in Islam and the life of Muslims in all Muslim nations. Muslims without a shred of evidence consider Allah to be the creator of all things in the universe. The Muslims’ God, Allah, occupies the most sanctified place in Islam. Knowing or understanding who or what Allah is, therefore, is critical to the understanding of Islam.
There are great many discussions and views on the origin of this term and its eventual adoption by Muhammad as referring to the creator of the universe.
Long before Islam, Arabs of the greater Arabian Peninsula worshiped, in the broadest sense of the term, the sun, the moon, and the stars. The root word for Allah is Ela, a term used by Yemeni Arabs for god and in reference to the moon. In addition, there were numerous other Ela. Allah is thought to be a compound word composed of al (the) and Ela.
It was Yemeni Arabs who migrated to the present Saudi Arabia who took the concept and the term Ela with them and propagated it. This is the root of the first part of the Muslim profession of faith, La ilah illaAllah, “there is no God but Allah.”
via Allah and Islam